fix(exporting): F01 路径完整验证前不登记清理目标，清理不跟随符号链接

- 根目录/用户目录/目标文件先通过 lstat 完整校验（拒绝符号链接与非目录），
  校验失败时不删除任何磁盘文件（此前父目录为链接时会沿链接 unlink 同名文件）
- 失败清理只处理本次登记的受控文件，并通过 O_NOFOLLOW 目录描述符删除；
  目录被换成符号链接时拒绝删除，保留任务编号与路径的对应关系
- 测试覆盖：用户目录链接、目标链接、运行中目录被换成链接（受保护文件字节不变）、
  失败后正常重试成功

tests/test_formula_export_reliability.py

@@ -198,6 +198,114 @@ def test_formula_export_worker_records_path_preparation_failure(workspace, setti
     assert job.status == "failed"
 
 
+def _queued_job(ds):
+    return FormulaExport.objects.create(
+        dataset=ds,
+        user=ds.created_by,
+        context={"start": "2026-08", "end": "2026-08", "comparison": "yoy", "group": "department"},
+    )
+
+
+def test_failed_export_rejects_symlinked_user_directory_without_touching_files(workspace, settings):
+    from finance.exporting import process_formula_export
+
+    ds, *_ = workspace
+    job = _queued_job(ds)
+    root = Path(settings.MEDIA_ROOT) / "formula-exports"
+    root.mkdir(parents=True)
+    protected = Path(settings.MEDIA_ROOT) / "protected"
+    protected.mkdir()
+    victim = protected / f"{job.pk}.xlsx"
+    victim.write_bytes(b"protected workbook")
+    (root / str(job.user_id)).symlink_to(protected, target_is_directory=True)
+
+    with pytest.raises(ValueError, match="符号链接"):
+        process_formula_export(job.pk)
+
+    job.refresh_from_db()
+    assert job.status == "failed"
+    assert not job.path
+    assert victim.read_bytes() == b"protected workbook"
+    assert list(protected.glob("*.tmp")) == []
+
+
+def test_failed_export_rejects_symlinked_target_and_keeps_link(workspace, settings):
+    from finance.exporting import process_formula_export
+
+    ds, *_ = workspace
+    job = _queued_job(ds)
+    user_dir = Path(settings.MEDIA_ROOT) / "formula-exports" / str(job.user_id)
+    user_dir.mkdir(parents=True)
+    protected_file = Path(settings.MEDIA_ROOT) / "protected.xlsx"
+    protected_file.write_bytes(b"protected workbook")
+    (user_dir / f"{job.pk}.xlsx").symlink_to(protected_file)
+
+    with pytest.raises(ValueError, match="普通文件"):
+        process_formula_export(job.pk)
+
+    job.refresh_from_db()
+    assert job.status == "failed"
+    assert protected_file.read_bytes() == b"protected workbook"
+    assert (user_dir / f"{job.pk}.xlsx").is_symlink()
+    assert list(user_dir.glob("*.tmp")) == []
+
+
+def test_failed_export_refuses_cleanup_when_parent_directory_swapped_to_symlink(
+    workspace, settings, monkeypatch
+):
+    from finance.exporting import process_formula_export
+
+    ds, *_ = workspace
+    job = _queued_job(ds)
+    root = Path(settings.MEDIA_ROOT) / "formula-exports"
+    user_dir = root / str(job.user_id)
+    user_dir.mkdir(parents=True)
+    protected = Path(settings.MEDIA_ROOT) / "protected"
+    protected.mkdir()
+    victim = protected / f"{job.pk}.xlsx"
+    victim.write_bytes(b"protected workbook")
+
+    def swap_and_fail(*args, **kwargs):
+        args[-1].write(b"partial")
+        user_dir.rename(user_dir.with_name("swapped"))
+        user_dir.symlink_to(protected, target_is_directory=True)
+        raise ValueError("controlled failure after path validation")
+
+    monkeypatch.setattr("finance.exporting.build_formula_workbook", swap_and_fail)
+    with pytest.raises(ValueError, match="controlled failure"):
+        process_formula_export(job.pk)
+
+    job.refresh_from_db()
+    assert job.status == "failed"
+    assert victim.read_bytes() == b"protected workbook"
+    assert user_dir.is_symlink()
+    # 本次创建的临时文件残留在被换名的原目录中，由清理命令按保留期回收。
+    assert list((root / "swapped").glob(f"{job.pk}.*.tmp"))
+
+
+def test_formula_export_retry_after_failure_succeeds(workspace, monkeypatch):
+    from finance.exporting import process_formula_export
+
+    ds, *_ = workspace
+    job = _queued_job(ds)
+
+    def fail(*args, **kwargs):
+        args[-1].write(b"partial")
+        raise ValueError("controlled export failure")
+
+    monkeypatch.setattr("finance.exporting.build_formula_workbook", fail)
+    with pytest.raises(ValueError, match="controlled export failure"):
+        process_formula_export(job.pk)
+    monkeypatch.undo()
+
+    stats = process_formula_export(job.pk)
+    job.refresh_from_db()
+    assert stats["rows"] == 5
+    assert job.status == "succeeded"
+    assert Path(job.path).read_bytes()[:2] == b"PK"
+    assert not list(Path(job.path).parent.glob(f"{job.pk}.*.tmp"))
+
+
 def test_formula_workbook_rejects_unclassified_label_collision(workspace):
     from decimal import Decimal